Structural foam plastics are being developed for appliance, automotive and furniture uses providing lower cost, light-weight products having high utility. Such products are molded from compositions that will foam under molding conditions wherein the gas liberating agent will provide gases in situ, at elevated molding temperatures, that blow the molded composition reducing its density by 2 to 50%.
Such foamable moldable compositions will fill injection molds readily, however, the set-up or cooling time in the mold has been longer than for conventional molding causing molding costs to be higher, offsetting the lower material cost advantages.
It is the objective of the present invention to provide foamable polyblend compositions that can be foamed and molded simultaneously with shorter cycles than conventional foamable polymeric materials.
U.S. Pat. No. 3,268,636 discloses a process for the injection molding of foamed plastic articles, teaching the general concept of molding foamable plastic using thermoplastics and various blowing agents.
The present invention relates to moldable foamable polymeric compositions of relative high density that can be used as structural tough molded articles for furniture, appliances and automotive. Styrenic plastics such as styrene-acrylonitrile copolymers and rubber reinforced polymers (ABS) are used conventionally for such molded articles having high modulus and toughness along with excellent melt flow properties for molding. Such materials when foamed with aliphatic blowing agents are plasticized with the blowing agents, hence, lose modulus and more importantly have longer set-up times or molding cycles.
Beyond molding, the invention also relates to foamable polymeric compositions that can be extruded into various profiles such as sheet, conduits, plank, etc., that can have foamed thick sections which have a tendency to lose dimensional stability and/or collapse if the blowing or foaming system is not optimum for the polymeric composition.
It is known to foam alkenyl aromatic polymers with hydrocarbons or other volatile fluid foaming agents that boil below about 100.degree. C. Such foaming systems have used nucleating systems to insure fine cell size structure. Such nucleating systems include compounds that will decompose to form carbon dioxide and nitrogen, e.g. bicarbonates activated by organic acids or various azo compounds. Such nucleating systems, however, have not been found to be efficient as blowing systems for alkenyl aromatic polymers and have been used with hydrocarbons.
U.S. Pat. No. 3,960,792 has disclosed blowing systems for polyalkenyl aromatic polymer such as polystyrene and finds that carbon dioxide systems will foam polystyrene, however, foam collapse in thick sections causes loss of dimensional stability, hence, certain halogenated hydrocarbons are used.
U.S. Pat. No. 3,344,092 discloses mixed carbonate and bicarbonate salts used with organic acids and hydrocarbons will foam polystyrene, however, the ratio of carbonates, bicarbonates and organic acids are critical to forming fine cell structures. Carbonates or bicarbonates used alone with organic acids given nonuniform large cell foams.
The prior art then discloses that foaming systems for a particular polymeric composition are not readily adapted to other polymer systems and that all foaming agents within a class are not mere equivalents even for the same polymeric composition.
It has been found, unexpectedly, that polybasic acids have the ability to foam alkenyl aromatic-alkenyl nitrile polymers in thick sections without collapse of the cells maintaining a fine, closed cell, structure having superior dimensional stability.